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Evaluation of Transport Parameters in MoS/Graphene Junction Devices Fabricated by Chemical Vapor Deposition Young Chul Kim, Van Tu Nguyen, Soonil Lee, Ji-Yong Park, and Yeong Hwan Ahn ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16177 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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ACS Applied Materials & Interfaces

Evaluation of Transport Parameters in MoS2/Graphene Junction Devices Fabricated by Chemical Vapor Deposition

Young Chul Kim, Van Tu Nguyen, Soonil Lee, Ji-Yong Park,* and Yeong Hwan Ahn*

Department of Physics and Department of Energy Systems Research, Ajou University, Suwon, 16499, Korea

*

Corresponding Author: [email protected] (JYP); [email protected] (YHA)

Keywords: MoS2, graphene, junction, diffusion length, mobility, scanning photocurrent microscopy, femtosecond pump-probe spectroscopy

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ABSTRACT: We demonstrated imaging of the depletion layer in a MoS2/graphene heterojunction fabricated by chemical vapor deposition and obtained their transport parameters such as diffusion length, lifetime, and mobility by using scanning photocurrent microscopy (SPCM). The device exhibited n-type operation, which was determined by the MoS2 layer with lower mobility. SPCM revealed the presence of the depletion layer at the heterojunction, whereas graphene provided an excellent electrical contact for the MoS2 layer without resulting in rectifying behavior, even if they were anchored within a very short range. The polarity of the photocurrent signal switched when we applied a drain–source bias voltage, from which we extracted the potential barrier at the junction. More importantly, biasdependent SPCM allowed us to simultaneously record the diffusion lengths of both majority and minority carriers for the respective MoS2 and graphene layers. By combining the diffusion lengths with the lifetimes measured by femtosecond SPCM, we determined the electron and hole mobilities in each layer, from which we found that the electron mobility (160 cm2 V-1 s-1) was higher than the hole mobility (80 cm2 V-1 s-1) in MoS2, whereas the hole mobility (15000 cm2 V-1 s-1) was relatively higher in graphene.

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1. Introduction Two-dimensional crystalline materials1 such as graphene2 and molybdenum disulfide (MoS2) have attracted much interest for their potential use in future-generation electronic,3-8 optoelectronic9-14 and sensing devices15-16 because they possess unprecedented optical and electric properties.17-18 For example, MoS2 exhibits high on–off ratio and n-type properties regardless of the number of layers.3 On the other hand, graphene is a zero-band-gap material but has high mobility, which is crucial for the development of high-speed electronic devices.19 In particular, heterojunctions created when we combine these two-dimensional materials appear to be a promising solution to the development of novel functional devices such as fast and sensitive photodetectors, logic circuits, and memory chips.20-23 There have been various studies over the past few years on MoS2/graphene heterostructured devices.10,20-22,24-31 In particular, MoS2/graphene heterostructures with high on-off ratios have been found to exhibit rectifying behavior and behave as p–n junction diodes, and the diode-like behavior has made it a candidate for application in next-generation integrated circuits.25,32 More recently, through the assembly of atomically sharp MoS2/graphene junctions, two-dimensional atomic circuitry of an inverter has been proposed.20 In order to optimize the device performance based on these novel heterostructures, it is crucial to address the localized electronic properties of MoS2/graphene; however, the heterojunctions based on these novel two-dimensional materials have not been visualized directly, especially in conjunction with an evaluation of the overall device performance. Moreover, previous studies focused on devices in which the exfoliated MoS2 film had been transferred onto the graphene layer, and there was a large lateral overlap area between the two-dimensional materials. In this study, we fabricated devices with a MoS2/graphene heterojunction based on a chemical vapor deposition (CVD) technique in which the edge of graphene was used as a

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nucleation site for the MoS2 growth. We determined the energy band structure of the devices with the sharp MoS2/graphene junction by using scanning photocurrent microscopy (SPCM) in conjunction with an evaluation of the overall device performance. In addition, the diffusion lengths of carriers were considered by recording the photoelectric response near the junction area, and we were able to estimate the electron and hole mobilities in the graphene and MoS2 films, respectively. Finally, combined with carrier-lifetime measurements based on the femtosecond SPCM technique, we were able to determine the carrier lifetime and mobility in each layer.

2. Results and discussion The devices with a MoS2/graphene lateral heterojunction were fabricated by a CVD technique. The fabrication began with synthesizing graphene which was transferred to a silicon substrate.33 Since MoS2 has the tendency to grow on the edge or wrinkles of graphene, the patterned graphene layer was used as an anchor for MoS2 synthesis in another atmosphere-pressure CVD chamber, as shown in the illustration in Figure 1(a). To fabricate devices with a MoS2/graphene junction, we defined the Cr and Au electrodes (thickness: 5 and 45 nm, respectively) such that the junction was located in the middle of the channel, as schematically shown in Figure 1(b). For SPCM measurements, the individual devices with MoS2/graphene junctions were illuminated using a diffraction-limited laser focal spot, while the electric current was recorded as a function of the laser spot position, as described in the literature.34-37

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Figure 1 (a) Schematic illustration of MoS2 growth in a CVD chamber, with the graphene edge used as a nucleation site. (b) Schematic of a device with a MoS2/graphene heterojunction. (c) AFM image of the MoS2/graphene device. (d) Raman spectra measured at position A (black line) and B (red line); the inset shows the position-dependent height extracted from the AFM image in (c).

A representative atomic force microscope (AFM) image for the MoS2/graphene heterojunction devices is shown in Figure 1(c). The device had a channel width of 14 µm and a length of 13 µm. The junction region is clearly shown in the middle of the channel between the graphene (denoted by A) and the MoS2 (denoted by B) regions. The step height between the graphene and MoS2 was found to be 0.8 nm, as shown in the inset of Figure 1(d), indicating that a bilayer MoS2 has been synthesized. The Raman spectra shown in Figure 1(d) confirm the presence of the individual layers, and hence, the formation of the junction in the middle of the channel. First, it is obvious that only the MoS2 layer was present in the B region (red line) because we can clearly see the E12g (393 cm-1) and A1g (415 cm-1) peaks, whereas no graphene-related peaks were found. The frequency difference between the two

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peaks yields ~ 22 cm-1 for most of devices we tested, indicating the formation of bilayer MoS2.38 This is consistent with the AFM image and the fact that the higher density of nucleation centers at the edge could create multiple MoS2 layer.20 Conversely, in the A region (black line), we could identify the strong graphene-related peaks, i.e, the G peak at 1600 cm-1 and the 2D peak at 2700 cm-1. However, the MoS2 Raman peaks also appear in the A region, whereas the peak is weaker than that obtained in the B region. This is a strong indication that there were MoS2 flakes on the graphene films; however, it seems that they did not form a continuous film, and hence, did not contribute to the carrier conduction in the graphene layer. We also want to point out that the relatively weak 2D peak of the graphene layer was due to the effects of strain induced by the MoS2 flakes on the graphene film.39 It is confirmed from the AFM image that the graphene was a single layer in the first place before it is used as part of the junction device (Supporting Information Figure S1).

Figure 2 (a) Curve of current (I) versus drain–source voltage ( VDS ) curve measured at various values of gate voltage ( VG ). (b) Plot of current as a function of VG at VDS = 100 mV . (Inset) Semi-logarithmic plot of

I − VG at VDS = 100 mV .

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The device’s transport properties are shown in Figure 2. Figure 2(a) shows the current versus drain–source voltage ( I − VDS ) plots at different gate bias voltages ( VG ). It is clear that the device conductance increased with increasing positive values of VG , which is also clearly demonstrated in the I − VG curve (for VDS = 100 mV ) shown in Figure 2(b). In other words, the device exhibited clear n-type operation, and it was virtually turned off when VG < 0 V . This suggests that the switching behavior was governed by the MoS2 channel, because of the low mobility of the MoS2 film relative to that of graphene. Weak rectifying behavior was also observed in the I − VDS curves in Figure 2(a), which can be attributed to the formation of Schottky barrier at the MoS2/metal contact, as will be discussed later.29

Figure 3 (a) Reflection image of the device. (b) Representative SPCM image of the MoS2/graphene device at

VDS = VG = 0 V . (c) SPCM signals taken along the channel of the device shown in (b) within the VDS range of -200 to 200 mV. The polarity of the current changed at the flat band voltage of Vf ≈ 45 mV . The panel on the left shows an illustration of the electronic band alignment.

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It has been demonstrated that the SPCM is an ideal tool for imaging localized electronic band structures such as metallic contacts, interfaces, and defects.34,36-37,40-42 In our study, the SPCM was used for imaging the electronic band structure formed at the MoS2/graphene junction and also for obtaining the transport parameters such as the diffusion length and mobility of each layer. The reflection image of the device is shown in Figure 3(a). Obviously, the bright MoS2 layer can be clearly identified along with the region of the metal electrodes (shown in red), whereas the graphene layer cannot be clearly distinguished in the image owing to the low reflectance. Figure 3(b) shows an SPCM image of the device. We used very weak light power of 53 µW, in which the thermoelectric current is suppressed (Supporting Information Figure S2). We observed strong photocurrent signals along the junction between the MoS2 and graphene layers, which is a strong indication that the depletion layer was formed at the heterojunction. The negative signal represented band-bending formation, in which an electron (hole) was transported to the source (drain) electrode. We also noticed that the metal contact signal was not pronounced for both source and drain electrodes, which was an indication that the potential drop would be dominant at the junction rather than at the metal contact regions. The primary goal of this study is to obtain information on the localized band as a function of the applied bias, VDS . It was expected that the strength and direction of band bending in the depletion layer would change with VDS . Figure 3(c) shows the SPCM signals ( ISPCM ) recorded along the channel, as a function of position (x axis) and VDS (y axis). The signal showed a large negative value when VDS = −200 mV and decreased until its polarity was reversed and positive values were obtained when VDS > 45 mV . The polarity switching can be understood in terms of the band-bending changes, as illustrated in the left panel of Figure 3(c). In particular, for the positive bias with VDS > 45 mV , the electrons will be transported

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toward the graphene layer before it is annihilated through recombination, whereas it is the holes that are transported toward the graphene for VDS < 45 mV . The measurement of VDS under the flat band condition gave a rough estimate of the potential barrier height between the layers, yielding Vf ≈ 45 meV when the metal contact resistance was ignored.

Figure 4 (a) I − VDS curves of device with MoS2/graphene heterostructure to show the rectifying behavior at different gate bias voltages ( VG ). (b) SPCM signal as a function of the position along the channel (x axis) and

VDS (y axis). (c) I − VDS curve of the device in (a) after annealing, without exhibiting any rectifying behavior. (d) SPCM signal as a function of the position along the channel (x axis) and VDS (y axis) after annealing.

As mentioned earlier, the potential drop primarily occurred at the junction, so the contribution from the metal contact resistance was negligible. Conversely, when the metallic contact was poor, a considerable potential drop occurred at the metal contacts. Figure 4(a)

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shows an I − VDS curve of another MoS2/graphene heterojunction device that exhibited strong rectifying behavior ( VDS = 200 mV ). Interestingly, SPCM signals were found at the MoS2/metal contacts as well as the heterojunction. In particular, the polarity of ISPCM changed mostly at the MoS2/metal junction when it was plotted as a function of VDS , as shown in Figure 4(b). This indicates that the resistance at the MoS2/metal junction was comparable to or higher than that of the MoS2/graphene junction. We note that it has been shown previously that the SPCM tends to be dominated by the metallic signals when the device shows a rectifying behavior due to the formation of the Schottky contacts or from relatively poor metal contacts.43 After annealing, the rectifying behavior of this other device disappeared, as shown in the

I − VDS curve of Figure 4(c), which implies that the Schottky contact disappeared. The annealing was carried out under a flow of Ar gas at 300°C for 15 min. Surprisingly, ISPCM of the annealed device (Figure 4(d)) was dominated by the MoS2/graphene heterojunction, and its polarity changed from negative to positive as VDS increased, as in Figure 3 for the first device. This occurred because of the reduced metal contact resistance, resulting in the application of VDS predominantly at the MoS2/graphene junction. We found the similar behavior from six different devices (out of eight devices). This result strongly suggests that graphene provided an excellent electrical contact for the MoS2 layer without resulting in rectifying behavior, even if they were anchored within a very short range.31 This is also consistent with the measurement of the relatively low potential barrier at the heterojunction, as shown in Figure 3(c). We also noticed that the device conductance was not improved by annealing because our annealing conditions likely induced damage on the MoS2 surface. However, advanced techniques such as the laser-induced annealing could be useful for finding an optimal condition without causing the damage to the devices.44

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From the data shown in Figure 3, we extracted the carrier diffusion length of both the graphene layer and the MoS2 layer near the junction. The SPCM technique has been widely used for determining the diffusion length in various nanoscale devices, including solar cells.35 The range of the SPCM signals from an interface is the measure of the carrier diffusion length. The ISPCM signals decayed exponentially as the focused laser spot moved away from the junction, as shown by the line profile of the plot in Figure 5(a), which was extracted from Figure 3(c).35 More importantly, it is clear that the diffusion length was much longer toward the graphene layer than the MoS2 layer, which reflects the higher mobility in graphene when compared to that in MoS2. We extracted the electron and hole diffusion lengths by fitting the data in Figure 5(a) with ISPCM ∝ exp(− x / L) , where x is the laser spot position and L is the diffusion length.35 The results are shown in Figure 5(b) as functions of VDS , for the respective graphene and MoS2 layers. In general, the electric-field applied parallel to the carrier motion strongly modifies the diffusion processes.35 However, in our case, the diffusion length does not change significantly with VDS when it is away from the flat-band conditions (i.e., for large

VDS − Vf ). This is likely because the voltage drop occurs predominantly at the junction area, resulting in the relatively weak field-driven effect outside the depletion layer. Here, we fitted the data when the laser spot position is away from the junction area, in other words, far from the depletion layers. On the other hand, the diffusion lengths were found to be small near the flat band condition (i.e., at VDS = Vf ) in which ISPCM is very weak and the decay constant was obtained rather close to the junction area. Therefore, the decay length measured near the flat band conditions could be more influenced by the presence of the depletion layer, rather than the diffusion processes.

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Focusing on the MoS2 layer, when VDS < Vf , the hole carriers (i.e., the minority carriers in the MoS2 layer) contributed to the observed photocurrent, as schematically illustrated in Figure 5(c), from which we could obtain the hole diffusion length ( Lh(MoS2 ) ) in the MoS2 layer. Conversely, when VDS > Vf , the electron carriers (i.e., the majority carriers in MoS2) contributed to the photocurrent, from which we could obtain the electron diffusion length ( Le(MoS2 ) ). The electron and hole diffusion lengths in the MoS2 layer yielded Le(MoS2 ) = 0.34 ± 0.042 µm and Lh(MoS2 ) = 0.24 ± 0.021 µm, respectively, for the electron and hole carriers, when the saturation condition was reached for VDS > 100 mV . The electron diffusion length was higher than the hole diffusion length, which has not been reported in MoS2 devices exhibiting typical n-type behavior. This was possible because our technique enabled us to simultaneously measure the diffusion length for the majority and minority carriers, even in unipolar devices. We also note that the MoS2 diffusion length (~ 300 nm) which is shorter than the laser spot size could be successfully addressed by fitting the data away from the junction area (Supporting Information Figure S3). Conversely, in the case of graphene, the diffusion lengths of the electron (minority) and hole (majority) carriers were similar to each other, yielding Le(Gr) = 0.9134 ± 0.040 µm and Lh(Gr) = 1.18 ± 0.216 µm. The diffusion length of the graphene layer tends to be large for the devices with higher conductance, whereas some of devices with poor conductance exhibit very short diffusion length (less than 300 nm) as can be found in Figure 4. Interestingly, the diffusion lengths did not change noticeably with the gate bias VG for both layers, although VG modifies the overall device conductance significantly (as shown in Figure 2). In other words, the diffusion process (or the carrier mobility) does do not seem to be influenced by the carrier concentration that varies with VG within our experimental condition, which necessitates future investigations.

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Figure 5 (a) Normalized SPCM current as a function of position near the MoS2/graphene junction for VDS = 0.2 and –0.2 V, extracted from Figure 4; the solid lines represented the fitted data. (b) Electron and hole diffusion lengths as functions of VDS extracted from (a) for the graphene and MoS2 layers. Solid lines are guide to the eye. (c) Principle of the diffusion length measurements for both minority ( VDS < Vf ) and majority ( VDS > Vf ) carriers in MoS2.

By using the expression for the diffusion length, L = Dτ and the Einstein relation, D = µ kBT / e , where D is diffusion constant, τ is carrier lifetime, µ is the mobility, kB is

Boltzmann’s constant, T is the absolute temperature,35 the diffusion lengths measured for the graphene and MoS2 layers could give us information on the carrier mobilities. Therefore, knowledge of the carrier lifetime would allow us to obtain transport parameters such as carrier mobilities.

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Figure 6 (a) Schematic illustration of the femtosecond SPCM measurement setup. (b) ∆I pr as a function of the position along the channel (x axis) and time-delay ( τ delay ) (y axis) for the device shown in Figure 6. (c) Normalized ∆I pr along the red (blue) dashed line for the lifetime measurements in the graphene (MoS2) layers.

We adopted the femtosecond photocurrent measurement technique to measure the carrier lifetimes, and the results are shown in Figure 6.45 As schematically illustrated in Figure 6(a), we monitored the photocurrent generated just by the femtosecond probe pulse ( I pr ) by using the lock-in technique. Figure 6(b) shows a plot of ∆I pr = I pr − I pr (off) as a function of position in the channel (x axis) and the time delay (y axis), where I pr (off) is the probe photocurrent

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generated without the pump laser. The position of the MoS2/graphene junction is marked by an arrow. ∆I pr is strong at the MoS2/graphene junction and decays with increasing time delay. In general, the carrier lifetime measured at the depletion region is strongly influenced by the escape phenomenon.46 Therefore, we measured the decay times of graphene and MoS2 individually, far from the depletion region, i.e., by taking ∆I pr along the dashed lines (red for graphene, and blue for MoS2) shown in Figure 6(b). By fitting the plot of ∆I pr as a function of τ delay (Figure 6(c)), we obtained time constants (τ c ) of 37 ± 3.2 ps and 280 ± 75 ps, for graphene and MoS2, respectively.47-48 The measured carrier lifetime in the MoS2 layer is thus in good agreement with previous reported values.48,49 On the other hand, the lifetime for the graphene is larger than the reported values of less than 5 ps.50-51 This can be attributed to the carriers generated in the MoS2 flakes that were placed on top of the graphene layer, as evidenced by the Raman signals shown in Figure 1(d). It should be noted that similar behavior has been reported for graphene/MoS2 nanocomposites.52 Finally, the electron mobility ( µe ) and hole mobility ( µ h ) were obtained from the results shown in Figures 5 and 6, which yielded µe = 8800 cm2 V-1 s-1 and µ h = 15000 cm2 V-1 s-1 for graphene. Similarly, the results for MoS2 yielded µe = 160 cm2 V-1 s-1 and µh = 80 cm2 V-1 s-1 for the electron and holes, respectively. The electron mobility in MoS2, which was higher than the hole mobility, has not been reported before for MoS2 devices with unipolar ntype operation.34,35 We also note that the MoS2 mobility is much higher than the field-effect mobility (~ 2 cm2/Vs) extracted from the transconductance as shown in Figure 2.29 This is because the field-effect mobility is influenced by various factors such as the metal contacts, grain sizes, surface charges, ripples, local defects, and so on. Conversely, our technique provides a unique tool for addressing the localized transport parameters without being

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obscured by the factors limiting the carrier transport. Therefore, the device with the sharp heterojunction provided an ideal platform for very accurate measurements of the transport parameters because, without the heterojunction, the SPCM signals would have been dominated by the general signals from the metals. Our work will open a door for understanding the transport phenomenon in two-dimensional crystalline materials and at the interfaces between them, which will help to improve device performances.

3. Conclusion In conclusion, we demonstrated the imaging of the depletion layer in a lateral MoS2/graphene heterojunction and obtained their transport parameters such as the diffusion length and the mobility by using the SPCM technique. The device exhibited n-type operation determined by the MoS2 layer because the mobility in MoS2 was lower than that in graphene. SPCM revealed that the depletion layer is formed at the heterojunction; however, graphene provided an excellent electrical contact for the MoS2 layer without resulting in rectifying behavior, even if they were anchored within a very short range. Conversely, the rectifying behavior in the transport measurements originated from the formation of the Schottky contact at the MoS2/metal contact, which could be controlled by annealing. The SPCM signal correlated well with the overall device performance. From the polarity switching behavior of the SPCM signals, we determined the potential barrier had a value of 45 meV at the MoS2/graphene junction. Bias-dependent SPCM allowed us to simultaneously measure the diffusion lengths of both majority and minority carriers for both layers, even if the device exhibited unipolar (n-type) behavior. To obtain the carrier lifetimes, we also performed femtosecond SPCM, and by combining the results with the diffusion lengths, we determined the electron (160 cm2 V-1 s-1) and hole (80 cm2 V-1 s-1) mobilities in the MoS2 layer.

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METHODS Fabrication of MoS2/graphene devices The fabrication began with synthesizing graphene on a copper foil using methane and hydrogen gases,53 and the synthesized graphene was transferred to a silicon substrate with a 300 nm thick SiO2 layer.33,54 In order to establish the graphene layer within the device channel, we used reactive ion etching (RIE) to pattern the graphene to a size of 100 × 14 µm2. The patterned graphene layer was used as an anchor for MoS2 synthesis in another atmosphere-pressure CVD chamber. By using perylene-3,4,9,10tetracarboxylic acid tetrapotassium salt (PTAS) as a seeding promoter, 3 mg of MoO3 powder, and 500 mg of sulfur powder, MoS2 was synthesized at 650°C for 10 minutes.55 To fabricate devices with a MoS2/graphene junction, we defined the Cr and Au electrodes (thickness: 5 and 45 nm, respectively) such that the junction was located in the middle of the channel. Finally, the MoS2 films were also patterned using the RIE process to narrow the channel width. SPCM setup Diode-pumped solid state (DPSS) laser at 532 nm was focused by an objective lens (Olympus Corporation; 100X, NA 0.8) and raster-scanned using galvanometer scanning mirrors (Thorlabs, Inc.). The full-width at half maximum (FWHM) of the focused laser spot was 520 nm. The laser amplitude was modulated by a photoelastic modulator (PEM) at 120 kHz, enabling us to achieve both rapid scanning and improved signal-to-noise ratio (SNR) simultaneously. The photocurrent signals were measured using a fast current pre-amplifier (Femto Messtechnik GmbH) and a lock-in amplifier (AMETEK, Inc.). Femtosecond SPCM setup Laser beams from a femtosecond Ti:Sapphire laser (centered at 400 nm with a repetition rate of 80 MHz and a pulse width of 60 fs) were divided into pump and probe beams, and τ delay is generated by a delay stage, which is located in the path of the pump beam. A pair of chirped mirrors is used to compensate for the positive dispersion that originates mainly from the objective lens. In this work, we scanned the position of the

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spatially overlapping pump and probe beams while varying τ delay . An optical modulator (Boston Micromachines Corporation) is used to optically modulate the probe pulse at 20 kHz to capture only the I pr signals and to exclude the photocurrents generated directly by the pump pulse. We also recorded I pr (off) (the probe photocurrent generated without the pump pulse) as a function of τ delay and extracted only the pump induced change from

∆I pr = I pr − I pr (off) .

ASSOCIATED CONTENT The Supporting Information is available free of charge. Evolution of graphene Raman spectra with MoS2 growth, Laser power dependence and thermoelectric current, MoS2 diffusion length vs focused beam profile (PDF).

AUTHOR INFORMATION *

Corresponding Author: [email protected] (JYP); [email protected] (YHA)

The authors declare no competing financial interests.

ACKNOWLEDGEMENTS This work was supported by the Midcareer Researcher Program (2017R1A2B4009177) through a National Research Foundation grant funded by Korea Government (MSIP) and by Human Resources Program in Energy Technology (20164030201380) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by Korea Government (MOTIE).

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